Where Did the Moon Come From?

The origin of the moon has been shrouded in mystery for ages. People only see one side of the moon at a time, and it affects humanity’s circadian rhythm, the push and pull of the tides, the opening and closing of flowers and the sex lives of animals. Facts about the moon phases have also led to folklore, such as not having surgery on the full moon or looking over your left shoulder at the moon for fear of bad luck.

Only in 1969 would humanity make the journey to the moon, and since then, only 11 others have made it back to the moon. Rocks and other samples were brought home to Earth, yielding surprising results over the years and leading to theories that haven’t wholly panned out.

Was the Earth broken apart? Was the moon a rogue object? Did the moon form by way of a planetary object car crash in space? Moon rock samples reveal that the satellite’s composition is similar to Earth, yet also different — debunking aspects of older theories and raising more questions. Slowly, pieces of each theory help us better understand the origin of the moon.

4 Theories About How the Moon Formed

1. A Broken Chunk of the Earth

The Spin-Off Theory: As the solar nebula spun in the early days, scientists theorized that a piece of the Earth broke away, and this became the moon.

However, scientists haven’t fully accepted this theory because it’s likely the Earth wasn’t spinning fast enough to cast off the moon, according to computer models.

2. Earth’s Little Sister

The Accretion Theory: When the Earth was in the process of accretion, scientists suggested that the moon could have assembled in the same way next to Earth. In fact, Galileo Galilei was among the first to suggest that the moon was similar to Earth, with rugged terrain, based on his observations through an advanced telescope.

There’s one problem: The physical composition of the moon and Earth differ enough to suggest that more’s at play. The moon has a density of 3.3 g/cc since it lacks iron, and the Earth has an average density of 5.5 g/cc due to its iron core. Interestingly, the moon has the same isotope composition of oxygen as the Earth, but materials from other areas of the solar system don’t. The moon has to be from the Earth’s neighborhood.

3. The Moon Followed Earth Home

The Capture Theory: Like a stray dog, the moon wandered the universe until one day it was pulled in by Earth’s gravitational force.

It’s a remotely plausible theory, but a series of events would have to come together very precisely. The moon just happened to be cruising through the solar system, when it suddenly decided to stop and orbit the Earth. It’s more likely that the moon would have crashed into Earth.

Again, since the oxygen isotopic composition is the same on Earth and the moon, the satellite had to originate in the neighborhood.

4. A Planetesimal Car Crash

The Cosmic Collision Theory: Two larger planets-in-progress, called planetesimals, crashed into each other in the solar nebula. The smaller planetesimal would have to be the size of Mars, which orbited around a just-forming Earth after the collision. Since the objects weren’t whole yet, the smaller one’s iron core was pulled to the newborn Earth due to gravity.

This theory is the most popular among scientists because it provides a very plausible explanation as to why the composition of the moon and Earth differ. Interestingly, the two only differ by a few parts in a million. Research modulations reveal that though most planetesimal collisions result in differing compositions, 20 to 40 percent are similar and are not at all rare in the universe.

The composition similarity arises as a result of what orbits the colliding bodies occupied during the point of impact. The compositions vary according to the amount of heat each object received — a colder object farther from the sun had a heavy isotope of oxygen. Since the Earth and moon have the same levels of isotopes, they were formed the same distance from the sun.

In the modulations, the last planetesimal that collided with the other most likely shared similar orbits, similar birthplaces and, therefore, similar compositions. The cosmic collision theory has the most scientific evidence so far, and who knows what further research will reveal?

The contradictions various theories faced have finally found a resolution: The compositions of the Earth and its moon are the same and yet different. Could it be as simple as the comparison of being two sides of the same coin, as are light to dark and warm to cold?

Nothing in science or life is ever black and white, and no matter superstition or scientific insight, the moon is beautiful to behold on a restless night.

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Megan Ray Nichols is a freelance science writer and science enthusiast. Her favorite subjects include astronomy and the environment. Megan is also a regular contributor to The Naked Scientists, Thomas Insights, and Real Clear Science. When she isn't writing, Megan loves watching movies, hiking, and stargazing.

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